The transmission of information by digital techniques is becoming the preferred mode of communicating voice and data information. High speed computers and processors, and associated modems and related transmission equipment, are well adapted for transmitting information at high data rates. Telecommunications and other types of systems are well adapted for transmitting voice information at data rates upwardly of 64 kilobits per second. By utilizing multiplexing techniques, transmission mediums are able to transmit information at even higher data rates.
While the foregoing represents one end of an information communication spectrum, there is also a need for providing communications at low or very low data rates. Underwater and low speed magnetic transmission mediums represent situations in which communications at low data rate are needed. The problems attendant with low data rate transmissions is that it is difficult to fully characterize an analog voice signal, or the like, with a minimum amount of data sufficient to accommodate the very low transmission data rate. For example, in order to fully characterize speech signals by pulse amplitude modulation techniques, a sampling rate of about 8 kHz is necessary. Obviously, digital signals corresponding to each pulse amplitude modulated sample cannot be transmitted at very low transmission bit rates, i.e., 200-1200 bits per second. While some of the digital signals could be excluded from transmission to reduce the bit rate, information concerning the speech signals would be lost, thereby degrading the intelligibility of such signals at the receiver.
Various approaches have been taken to compress speech information for transmission at a very low data rate without compromising the quality or intelligibility of the speech information. To do this, the dynamic characteristics of speech signals are exploited in order to encode and transmit only those characteristics of the speech signals which are essential in maintaining the intelligibility thereof when transmitted at very low data rates. Quantization of continuous-amplitude signals into a set of discrete amplitudes is one technique for compressing speech signals for very low data rate transmissions. When each of a set of signal value parameters are quantized, the result is known as scalar quantization. When a set of parameters is quantized jointly as a single vector, the process is known as vector quantization. Scalar and vector quantization techniques have been utilized to transmit speech information at low data rates, while maintaining acceptable speech intelligibility and quality. Such techniques are disclosed in the technical article "Vector Quantization In Speech Coding", Proceedings of the IEEE, Vol. 73, No. 11, Nov., 1985.
Matrix quantization of speech signals is also well-known in the art for deriving essential characteristics of speech information. Matrix quantization techniques require a large number of matrices to characterize the speech information, thereby being processor and storage intensive, and not well adapted for low data rate transmission. A significant degradation of the intelligibility of the speech information results when employing matrix quantization and low data rate transmissions.
When vector quantizing a signal for transmission, a vector "X" is mapped onto another real-valued, discrete-amplitude, N-dimensional vector "Y". Typically, the vector "Y" takes on one definite set of values referred to as a codebook. The vectors comprising the codebook are utilized at the transmitting and receiving ends of the transmission system. Hence, when a number of parameters characteristic of the speech information are mapped into one of the codebook vectors, only the codebook vectors need to be transmitted to thereby reduce the bit rate of the transmission system. The reverse operation occurs at the receiver end, whereupon the vector of the codebook is mapped back into the appropriate parameters for decoding and resynthesizing into an audio signal. While matrix quantization offers one technique for compressing speech information, the intelligibility suffers, in that one generally cannot discriminate between speakers.
From the foregoing, it can be seen that a need exists for a speech compression technique compatible with data rates on the order of 400 bits per second, without compromising speech quality or intelligibility. An associated need exists for a speech compression technique which is cost-effective, relatively uncomplicated and can be carried out utilizing present day technology.